/* Copyright (c) 2012, Linaro Limited
   All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions are met:
       * Redistributions of source code must retain the above copyright
         notice, this list of conditions and the following disclaimer.
       * Redistributions in binary form must reproduce the above copyright
         notice, this list of conditions and the following disclaimer in the
         documentation and/or other materials provided with the distribution.
       * Neither the name of the Linaro nor the
         names of its contributors may be used to endorse or promote products
         derived from this software without specific prior written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

/* Assumptions:
 *
 * ARMv8-a, AArch64
 */

#include <private/bionic_asm.h>

#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080

/* Parameters and result.  */
#define src1        x0
#define src2        x1
#define result        x0

/* Internal variables.  */
#define data1        x2
#define data1w        w2
#define data2        x3
#define data2w        w3
#define has_nul        x4
#define diff        x5
#define syndrome    x6
#define tmp1        x7
#define tmp2        x8
#define tmp3        x9
#define zeroones    x10
#define pos        x11

    /* Start of performance-critical section  -- one 64B cache line.  */
ENTRY(strcmp)
    eor    tmp1, src1, src2
    mov    zeroones, #REP8_01
    tst    tmp1, #7
    b.ne    .Lmisaligned8
    ands    tmp1, src1, #7
    b.ne    .Lmutual_align
    /* NUL detection works on the principle that (X - 1) & (~X) & 0x80
       (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
       can be done in parallel across the entire word.  */
.Lloop_aligned:
    ldr    data1, [src1], #8
    ldr    data2, [src2], #8
.Lstart_realigned:
    sub    tmp1, data1, zeroones
    orr    tmp2, data1, #REP8_7f
    eor    diff, data1, data2    /* Non-zero if differences found.  */
    bic    has_nul, tmp1, tmp2    /* Non-zero if NUL terminator.  */
    orr    syndrome, diff, has_nul
    cbz    syndrome, .Lloop_aligned
    /* End of performance-critical section  -- one 64B cache line.  */

#ifndef    __AARCH64EB__
    rev    syndrome, syndrome
    rev    data1, data1
    /* The MS-non-zero bit of the syndrome marks either the first bit
       that is different, or the top bit of the first zero byte.
       Shifting left now will bring the critical information into the
       top bits.  */
    clz    pos, syndrome
    rev    data2, data2
    lsl    data1, data1, pos
    lsl    data2, data2, pos
    /* But we need to zero-extend (char is unsigned) the value and then
       perform a signed 32-bit subtraction.  */
    lsr    data1, data1, #56
    sub    result, data1, data2, lsr #56
    ret
#else
    /* For big-endian we cannot use the trick with the syndrome value
       as carry-propagation can corrupt the upper bits if the trailing
       bytes in the string contain 0x01.  */
    /* However, if there is no NUL byte in the dword, we can generate
       the result directly.  We can't just subtract the bytes as the
       MSB might be significant.  */
    cbnz    has_nul, 1f
    cmp    data1, data2
    cset    result, ne
    cneg    result, result, lo
    ret
1:
    /* Re-compute the NUL-byte detection, using a byte-reversed value.  */
    rev    tmp3, data1
    sub    tmp1, tmp3, zeroones
    orr    tmp2, tmp3, #REP8_7f
    bic    has_nul, tmp1, tmp2
    rev    has_nul, has_nul
    orr    syndrome, diff, has_nul
    clz    pos, syndrome
    /* The MS-non-zero bit of the syndrome marks either the first bit
       that is different, or the top bit of the first zero byte.
       Shifting left now will bring the critical information into the
       top bits.  */
    lsl    data1, data1, pos
    lsl    data2, data2, pos
    /* But we need to zero-extend (char is unsigned) the value and then
       perform a signed 32-bit subtraction.  */
    lsr    data1, data1, #56
    sub    result, data1, data2, lsr #56
    ret
#endif

.Lmutual_align:
    /* Sources are mutually aligned, but are not currently at an
       alignment boundary.  Round down the addresses and then mask off
       the bytes that preceed the start point.  */
    bic    src1, src1, #7
    bic    src2, src2, #7
    lsl    tmp1, tmp1, #3        /* Bytes beyond alignment -> bits.  */
    ldr    data1, [src1], #8
    neg    tmp1, tmp1        /* Bits to alignment -64.  */
    ldr    data2, [src2], #8
    mov    tmp2, #~0
#ifdef __AARCH64EB__
    /* Big-endian.  Early bytes are at MSB.  */
    lsl    tmp2, tmp2, tmp1    /* Shift (tmp1 & 63).  */
#else
    /* Little-endian.  Early bytes are at LSB.  */
    lsr    tmp2, tmp2, tmp1    /* Shift (tmp1 & 63).  */
#endif
    orr    data1, data1, tmp2
    orr    data2, data2, tmp2
    b    .Lstart_realigned

.Lmisaligned8:
    /* We can do better than this.  */
    ldrb    data1w, [src1], #1
    ldrb    data2w, [src2], #1
    cmp    data1w, #1
    ccmp    data1w, data2w, #0, cs    /* NZCV = 0b0000.  */
    b.eq    .Lmisaligned8
    sub    result, data1, data2
    ret
END(strcmp)
